So, in the past I've never had any problems with this app. All benchmarks had 100% success rate. Yesterday I set up nginx to server static content and pass on other requests to apache. Now, if I have 1 concurrent user (-c 1) then everything is fine. But it seems the more concurrent users I have, the more failed requests I get. Not a lot, but maybe about 10 or 15 out of 350. They're "length", whatever that means. Visiting the website with a browser, I don't have any problems at all. How can I find out the cause of these failed requests?

These "length" failures just mean that the content length (amount of data served by your application) for some of the tries did not match up with the length of the first request. So, if ab got 100 bytes the first time. and then recevied 150 bytes the next 9 times, it would report back with 9 length failure.

If you are serving dynamic content, these errors are expected and can be safely ignored.

The answer by Marco Ramos is not correct, there is no reason to do anything like running netstat.